The present invention relates generally to so-called air-cushion vehicles (known also as hovercraft and surface-effect vehicles). A craft of this type can be propelled to travel in a wafted state over any of various different surfaces such as ground, water, ice, snow, and marshes. The craft is thus lifted by a cushion of air formed and maintained below the vehicle by pressurized air which is a portion of air blown by an engine driven propeller or fan mounted on the vehicle body or hull. The above mentioned surfaces function as pressure receiving surfaces against which the pressurized air acts.
More particularly, the present invention relates to the propulsion system including the fan of an air-cushion vehicle (hereinafter referred to as ACV), in which system the noise emitted by the fan is reduced to a minimum by a number of innovative measures and means of the present invention.
A typical example of an ACV of the character briefly described above, is illustrated in FIGS. 15 and 16 of the accompanying drawings. A greater part of the air blown by the fan is directed aft from the hull to function as the propulsive force or thrust to propel the hull forward. In general, the fan has a number of blades, which are encompassed around their tips by a cylindrical duct. This duct is spaced apart by a small clearance from the tips of the fan blades and is rigidly supported relative to the ACV body or hull.
In a conventional ACV, the inner diameter and the outer diameter of the duct are respectively constant from the duct inlet to the outlet, and the longitudinal section of the duct in its axial direction is rectangular. Furthermore, the chord length of each fan blade is constant in its spanwise direction i.e., from root to tip, or tapers to a narrow tip chord. Another feature of a conventional ACV is that the engine and the duct are respectively mounted independently on the ACV body. The duct is generally fabricated from a plate material such as thin steel plate, aluminum plate, or an FRP plate.
In the above described prior art, it has been possible to lift up the hull of an ACV and to cause it to travel stably over various kinds of surfaces. However, special measures have never been considered in the fan, the duct, and related parts with respect to reducing noise. Consequently, the noise generated from the fan and the duct which encompasses the fan to form an air stream path is transmitted to the cockpit. This noise annoys the driver and passenger of the vehicle.